1. Field of the Invention
The present invention pertains generally to the heating and thermal conditioning of glass sheets, and more particularly to an apparatus and a method of operation thereof for efficiently and relatively inexpensively producing flat, thermally-conditioned architectural glazing units of superior quality.
2. Description of the Prior Art
Glass for automotive and architectural purposes is generally produced as a continuous glass ribbon which is annealed and served into individual blanks of suitable dimensions for subsequent use. Such blanks to be fabricated into automotive glass parts are trimmed to a desired shape and then reheated to their softening temperature and bent to a desired configuration. The bent sheets destined for sidelite and backlite use, while still in a heated condition, are subjected to a rapid quenching or cooling so as to quickly lower their temperature below the strain point temperature and cause thermal tempering of the lite. Those bent sheets destined to be made into windshields are gradually cooled and annealed, and then laminated in pairs to a plastic interlayer material. Due to the nature of the industry, automobile glazing units of the same configuration can be produced at a high volume for an extended period of time. Consequently, the construction of elaborate relatively expensive facilities wherein glass sheets advance in succession continuously through heating, bending and thermal conditioning sections can be justified.
Tempered glass units for architectural purposes are produced in much smaller quantities and in less standardized dimensions. Thus, large expenditures to build and operate the relatively long heating furnaces and tempering and cooling sections of the type used for automotive production and necessary for continuous, straight through operation may not be justified. From an economic standpoint it is desirable to employ a facility having much shorter, and thus less expensive, heating and thermal conditioning sections within which the glass could be heated to a temperature above its strain point and then rapidly chilled below the strain point for achieving a suitable degree of thermal temper in the glass. However, heating and quenching of the glass are time dependent, and there are practical limitations to the minimum time interval within which the glass can be heated to the strain point temperature and then quenched and cooled to handling temperature. If the glass sheets are stopped within the heating furnace, or advanced therethrough at a very low speed to allow time for the sheets to reach the desired temperature throughout, they may tend to sag and bend between the lines of support provided by the spaced conveyor rolls. Extended contact along concentrated areas by the conveyor rolls may also result in optical defects on the glass surface known as roll marks.
In order to minimize the aforementioned conditions in the heating and tempering of glass in short facilities, it has been proposed to reciprocally convey a glass sheet load back and forth in an oscillatory manner within the heating chamber and/or quenching section. It is thus possible to achieve the necessary dwell time within the chamber without holding the glass in a static position for a significant time or conveying the glass at such a slow speed as to permit sagging of the glass in the unsupported area between adjacent conveyor rolls.
A number of proposals have been put forth heretofore for alleviating the aforementioned problems by conveying glass sheets in an oscillating manner during heating and/or thermal conditioning. The devices and methods disclosed in U.S. Pat. Nos. 1,856,669, 3,994,711, 4,297,121, 4,300,937, 4,591,374, 4,528,016, 4,617,043 and 4,816,055, for example, pertain to various designs and methods including oscillating or reciprocating movement of glass sheets. Reexamination Certificate B1 3,994,711, issued Jul. 3, 1990, contains an exhaustive list of patents dealing with the subject. Despite the extensive prior art, none of the disclosed systems has proven entirely successful in overcoming the aforementioned problems in producing thermally conditioned glass sheets of high quality within a relatively inexpensive facility.